This invention relates to a load lock-out control system for battery powered equipment which is particularly intended to save the batteries by locking out the load when a reduced charge condition of the batteries is detected. The invention is particularly useful in control systems for industrial lift trucks where the load which is locked out is the forklift motor load.
It is known to be very important in the operation of battery powered equipment to guard against excessive discharge of the batteries. Such excessive discharge can result in serious damage to the electric motors and control contactors (because of exposure to low voltage), as well as to the batteries. Furthermore, in the case of battery powered vehicles, the unexpected final depletion of the battery charge can leave the operator stranded at some distance from charging equipment or a source of fresh batteries. For this reason, it has become the custom to equip battery powered equipment such as industrial forklift trucks, and gold carts, and other vehicles, with battery state-of-charge meter systems, sometimes referred to as "battery fuel gauges". These state-of-charge meter systems may operate on various principles, including detection and integration of current supplied by the battery, detection of battery terminal voltage, or combinations of the two. These state-of-charge meter sytems may sometimes be referred to below as "battery state-of-charge monitoring circuits".
Two very satisfactory battery state-of-charge meter systems which may preferably be employed in conjunction with the present invention are exemplified by U.S. Pat. No. 4,193,026 for a "Method and Apparatus for Measuring the State of Charge of a Battery by Monitoring Reductions in Voltage" issued Mar. 11, 1980 to Eugene P. Finger and Eugene A. Sands and assigned to the same assignee as the present application, and patent application Ser. No. 321,671 filed on Nov. 16, 1981 by Eugene P. Finger for a "Battery State of Charge Metering Method and Apparatus" and assigned to the same assignee as the present application. Both of the above-mentioned patents relate to battery state-of-charge meters which operate on the principle of integrations of battery terminal voltage excursions under a voltage threshold level which are indicative of battery discharge events.
Unfortunately, the operator of the battery powered equipment often is inattentive to the battery state-of-charge meter, or overlooks or disregards the meter indications in his eagerness to proceed with the task at hand. For this reason, it has become generally recognized as desirable to reduce the drain on the battery by locking out the function of at least part of the equipment when the battery charge reaches a depleted level at which recharge is required. This is commonly referred to as a load lock-out function.
With a forklift truck, two electric motors are conventionally used, one for traction, and one for lifting loads with the lifting fork. Both of these motors are required for ordinary operations. However, only the traction motor is required for returning the forklift truck to a charging station. Accordingly, it has become the custom to have the battery state-of-charge monitor disable the lift motor (locking out that load) when the battery charge is depleted, preferably after having given the operator a warning signal. The operator then has no recourse but to use the traction motor to return to the charging station to obtain a recharge.
The forklift function is typically carried out with a hydraulic drive mechanism, in which the electric lift motor operates a hydraulic pump which causes the hydraulic lift drive to raise the load. When the desired height is reached, the lift motor is not longer required, and is typically switched off. When the forklift is to be lowered, that operation can be accomplished by simply opening a valve in the hydraulic lift drive without energizing the lift motor. Accordingly, the load and the forklift mechanism can be lowered even if the lift motor is locked out. However, in many systems, the lift motor can be locked out while it is operating ot lift the load. This means that the lifting operation cannot be completed, and the load often cannot be removed from the lift forks, and must be carried with the forklift truck back to the charging station. This can be very inconvenient when the load is consigned to a particular shipment.
For the above reasons, a better system is to synchronize the load lock-out function with the completion of any lifting operation which is in progress when the load lock-out function is called for. This synchronized operation may be referred to as a synchronous mode of load lock-out operation. The other mode of operation in which the load lock-out function is not synchronized with the normal switching off of the lift motor may be referred to as an asynchronous mode of operation. the synchronous mode of operation is very advantageous because it permits the operator to complete the lifting of the load and the subsequent depositing of the load before it is necessary to move the forklift truck back to the charging station.
One example of a battery monitor and a synchronous load lock-out device combination which is presently commercially available is a Curtis model 933/3 battery controller sold by Curtis Instuments Inc., Mt. Kisco, N.Y., U.S.A. That controller uses a silicon-controlled rectifier (SCR) as the load lock-out switching device. The SCR is used in a mode in which a constant DC enabling signal is supplied to the control electrode (gate) of the SCR as long as the battery charge is sufficient, and is removed when the battery charge is depleted. As long as the DC control voltage is available, the SCR will conduct current to a controller for the lift motor, whenever such current is called for. Furthermore, because of the inherent characteristics of the SCR, it continues to conduct an existing current even if the control voltage is removed because the battery charge has been depleted. However, once interrupted, the current cannot be reestablished through the SCR in the absence of the control voltage.
This system works well, except that it is essential for the operation of the system that the SCR be connected at a predictable position in the lift motor controller circuit so that the potential on the power electrodes of the SCR is predictable, and so that the potential on the control electrode may be properly set. This presents a substantial disadvantage, since the forklift trucks have different wiring arrangements, and it would often be convenient to interrupt the lift controller circuit at any selected position in that circuit, without being restricted to a particular designated position in the circuit.
It has also been found that the interconnection between the monitoring circuit and the lift motor controller circuit through the SCR may lead to troublesome voltage transients being carried from the power side of the system back into the battery state-of-charge monitoring circuit. These transients can sometimes cause difficulties since they may be interpreted as signal pulses within the monitoring circuit.
Accordingly, it is one object of the present invention to provide an improved synchronous load lock-out control system for battery powered equipment which permits interruption of virtually any selected portion of the contactor control circuit for locking out the load.
Another object of the invention is to provide an improved synchronous load lock-out control system for battery powered equipment in which an associated battery state-of-charge monitoring circuit is electrically isolated completely from the circuit which is locked out.
Further objects and advantages of the invention will be apparent from the following description and the accompanying drawings.